(Recasts with Ensign, Dodd, coalition comments, background)

By Kevin Drawbaugh

WASHINGTON, June 25 (Reuters) - A lone Republican lawmaker dug in his heels on Wednesday and vowed to block a major U.S. Senate housing market rescue bill until he gets his way on renewable energy tax credit legislation.

"I don't have any problem being an obstructionist when you're trying to do something really good," Sen. John Ensign of Nevada told reporters, leaving Democratic and Republican supporters of the housing bill frustrated.

"I regret that my colleague from Nevada feels as he does, but he's put in jeopardy a bill that could make a difference," said Connecticut Democratic Sen. Christopher Dodd, chief architect of the sweeping housing measure.

The bill would create a multibillion dollar fund to help hundreds of thousands of troubled homeowners refinance their costly, exotic mortgages into more affordable loans backed by the government, if lenders agreed to take a loss on the loans.

The legislation would overhaul regulation of Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research), the nation's largest mortgage finance companies, while also sending federal money to states and communities to buy and renovate foreclosed properties.

Some housing activists have questioned whether the housing bill would have much impact, even if it does get approved.

"Unfortunately, the legislation would prevent less than two months worth of foreclosures, falling far short of the more than two million that appear certain in 2008," the National Community Reinvestment Coalition said in a statement.